Project Summary Breast cancer is the most commonly diagnosed cancer in women, and more than 40,000 women in the US die each year of metastatic breast cancer. The estrogen receptor-? (ER?) and its ligand 17?-estradiol (E2) play critical roles in breast cancer. E2 promotes breast cancer progression by eliciting genomic effects in ER?- positive breast cancers by binding to its canonical receptor ER?66. However, triple negative breast cancers (TNBC) lack ER?66 and do not respond to hormonal therapy such as tamoxifen. It has been suggested that the novel splice variant ER?36 mediates rapid, non-genomic responses to E2. Previous studies from our lab showed that E2 activates sphingosine kinase 1 (SphK1), which produces the bioactive sphingolipid metabolite sphingosine-1-phophate (S1P). Secreted S1P in turn binds to its receptors leading to downstream signaling pathways and transactivation of EGFR important for breast cancer progression and metastasis. My preliminary results led me to propose the intriguing hypothesis that ER?36 is the membrane receptor required for E2- mediated SphK1 activation and rapid secretion of S1P, which regulates key non-genomic effects of E2 involved in hormone therapy resistance of breast cancer. I also suggest that increased expression of SphK1 and ER?36 play an important role in de novo and acquired endocrine resistance in metastatic breast cancer. Understanding the mechanisms responsible for de novo and acquired hormonal therapy resistance may provide clues to better treatments. The following specific aims will be pursued to test this hypothesis: (1) Establish the E2 receptor involved in activation of the SphK1/S1P axis in TNBC cells; (2) Determine the role of SphK1/S1P axis in in vitro and in vivo models of endocrine resistant breast cancer; (3) Examine the correlation of expression of SphK1 and ER?36 in endocrine resistant metastatic breast cancer patients and prognosis. This proposal will establish the role of the SphK1/S1P axis in non-genomic effects of E2 and in the regulation of endocrine resistance in metastatic breast cancer and will lay the foundation for further research on potential targeting of the SphK1/S1P pathway as an innovative therapeutic option to circumvent endocrine resistance and improve patient outcome.